The concentration of ten metals by the cultured bluegreen “picoplankter” Synechococcus sp. was investigated using radiotracer methodology in a series of laboratory experiments. Volume/volume concentration factors ranged from not significantly different from zero for Np and Cs to 106 for Sn, Hg, and Pu. As in eukaryotic algae, bioaccumulation of metals in Synechococcus sp. generally conformed with Freundlich adsorption isotherms. Given the ubiquity and abundance of the picoplankton and their high affinity for some metals, it is estimated that picoplankton-based food webs would represent a major route for the movement of particle-reactive metals in marine ecosystems.